A gas turbine engine includes a number of rotor sections axially aligned, each having a hub (or portion of a common hub) with a plurality of equally spaced rotor blades mounted on the hub. A shroud encompasses the blade tips with as little clearance as possible in order to minimize bypass flow of air and other gasses past the tips of the blades. The shroud is substantially coaxial, but it is very difficult to fabricate and maintain a shroud that is exactly round and located right at the blade tips, particularly with some flexing and heat expansions of the shroud during engine operations.
A common solution is to utilize a clearance sealing layer on the shroud that is abraded by the blade tips, thus producing a self-adjusting, relatively tight seal. For lower temperature sections, abradable coatings typically contain a soft metal with a soft non-metal component such as graphite, a polymer or boron nitride. In higher temperature sections a porous ceramic such as zirconia is used, such as described in U.S. Pat. No. 4,280,975. However, shroud materials, particularly ceramics, have a tendency to wear the tips of the blades. In the case of titanium blades, metallic friction against the shroud is a concern for fire.
Abrasive materials such as silicon carbide (SiC) have been provided on turbine blade tips to alleviate these problems, for example as taught in U.S. Pat. Nos. 4,492,522 and 4,802,828. SiC is considered good for the purpose because it is hard, has a high sublimation temperature and is oxidation resistant at turbine operating temperatures. However, SiC reacts with blade alloys at elevated temperatures, particularly nickel superalloys, to decompose the SiC and create deleterious by-products of silicides and free carbon. In U.S. Pat. No. 4,249,913 there is disclosed the use of SiC particles coated with alumina as a barrier against the blade alloy. However, SiC can react with alumina at high temperature in a reducing atmosphere to produce gaseous phases of SiO, CO and Al.sub.2 O. Also, significant differences in thermal expansion coefficients and thermal conductivities between SiC and a Al.sub.2 O.sub.3 layer make the layer susceptible to thermal cracking and subsequent reaction of the exposed SiC. Moreover, Al.sub.2 O.sub.3 has poor wetting by the blade alloy and is difficult to bond into it.
Boron nitride exists in several forms including cubic boron nitride ("cBN") which is very hard, second only to diamond. It has been used for abrasive particles on blade tips to cut ceramic outer air seal coatings, as taught in U.S. Pat. No. 5,704,759 and United Kingdom patent application publication No. GB 2 241 506 A. However, cBN oxidizes at the temperatures above about 850.degree. C. which are typical of the turbine sections utilizing ceramic shrouds.
An object of the invention is to provide a novel granular composition for application to tips of rotor blades in a gas turbine engine wherein the blade tips rub against an abradable ceramic or metallic layer on a shroud encompassing the rotor blades. Another object is to provide such a composition containing silicon carbide as abrasive granules. Yet another object is to provide such a composition in which silicon carbide granules have an improved layer of protection against reaction with blade alloy. A further object is to provide such a composition of silicon carbide having a protective layer with a further improvement for bonding into the blade alloy. Another object is to provide such a composition further containing a harder run-in material. An additional object is to provide a rotor blade with a blade tip containing such a novel composition.
Yet another object is to provide a process for bonding such a novel composition to a rotor blade tip.